Trapezoidal belt variable speed transmissions are commonly used on small vehicles such as snowmobiles, scooters or small cars. Such transmissions comprise a driving pulley, a trapezoidal belt and a driven pulley. The driving pulley is linked to an engine and the driven pulley is usually mechanically connected to ground traction means, such as wheels or tracks.
The main object of using a variable speed transmission is to automatically change the winding diameter of the trapezoidal belt around the driving and the driven pulleys in order to have a maximum torque at low speeds and a reasonable engine rotation speed at high speeds. The sides of the trapezoidal belt are, on each pulley, gripped between two opposite flanges wherein one is fixed and one is movable. At low speeds, the winding diameter of the driving pulley is small and the winding diameter of the driven pulley is maximum. As the rotation speed of the driving pulley increases, the movable flange of the driving pulley gets closer to the fixed flange and thus forces the trapezoidal belt to wind on a greater diameter. Since the length of the trapezoidal belt is not substantially stretchable, the trapezoidal belt exerts a radial force towards the center on the flanges of the driven pulley in addition to the tangential driving force. This radial force constrains the driven pulley to have a smaller winding diameter. Therefore, the movable flange of the driven pulley moves away from the fixed flange until the return force exerted by a spring counterbalances the radial force exerted by the trapezoidal belt. It should be noted at this point that a change in the load also produces a variation of the winding diameter of the pulleys, a greater load inducing a greater winding diameter of the driven pulley.
When the rotation speed of the engine decreases, the winding diameter of the driving pulley decreases and the radial force exerted by the trapezoidal belt decreases, thus allowing the driven pulley to have a greater winding diameter.
An example of such a variable speed transmission is disclosed in U.S. Pat. No. 3,266,330.
One of the drawbacks of conventional variable speed transmissions is that the driven pulley do not always set the trapezoidal belt at the maximum winding diameter when the vehicle is stopped very rapidly from a high speed, especially when an important brake force is applied on the wheels or tracks. The trapezoidal belt will then be stuck somewhere between the high speed position and the initial position. At restart, the ratio between the pulleys will not be optimum and the transmission will not deliver full torque.
The above-mentioned drawback is due to the physical limitations imposed by the use of a spring and cam system in the driven pulley to counterbalance the radial force exerted by the sides of the trapezoidal belt on the flanges. The spring is usually a helicoidal spring and is mounted between the movable flange and a fixed part. The cam is usually mounted around the spring and converts the torque exerted on the movable flange to axial gripping force on the belt.
In an ideal driven pulley, the gripping force on the belt at start is maximum because the belt pull is high to transmit engine torque on a small driver radius. At higher speed, the belt pull is lower and the gripping force must be reduced to maintain good efficiency.
In a conventional driven pulley, the spring and cam system provide a gripping force that reduces as speed increases, as long as the engine supplies power through the transmission. However, during braking, the engine absorbs power through the transmission. This unloads the cam and the gripping force is then supplied mainly by the spring. In this situation, the spring exerts less and less gripping force on the movable flange as it moves closer towards the fixed flange. This low gripping force is usually insufficient to force the belt towards a higher winding diameter during a rapid braking action. To generate enough gripping force, it would require a stronger spring. However, the gripping force would then be too high at high speed, reducing efficiency and increasing belt wear.